First, Sign in to the Amazon SNS console.

  1. In the left navigation pane, Go to ‘Subscription’ 
  2. In the left panel, click on Create Subscription
  3. Select the topic you just created for Topic ARN → Set "Protocol" to "HTTPS" → Enter Endpoint: https://(http://Force.com SiteBaseUrl)/services/apexrest/DragDropToCloud/S3ToSalesforce → Check "Enable raw message delivery" 
  4. Click on the Create Subscription button.
    1. (To get http://Force.com SiteBaseUrl → Setup → Sites → Edit for the site you created → Copy the link in Default Web Access)

PS: View Drag, Drop & Upload Files to Amazon S3, a Salesforce AppExchange app for moving files from Salesforce to Amazon S3: https://appexchange.salesforce.com/appxListingDetail?listingId=a0N3000000B5ZPqEAN